1. Field of the Invention
This invention relates to the field of semiconductor devices, and more specifically, to a process for forming contacts with vertical sidewalls, improved salicide and photoresist selectivity, and submicron dimensions.
2. Background Information
As semiconductor devices shrink ever smaller, so must the circuitry (components) such as contacts, plugs, vias, and interconnect lines. For example, in the current generation of semiconductor devices, the density of the circuitry (components) has become so compact that the dimensions of these components have decreased below submicron dimensions (&lt;1 micron).
A consequence of the smaller devices and the increasing density of the components on these devices is that it becomes increasingly more important to control the critical dimensions of these components. As the dimensions for these components become even smaller and spaced relatively closer, for example, less than 0.5 micron (&lt;0.5.mu.) and even less than 0.25 micron (&lt;0.25.mu.), the processes used to make such components become less reliable and are unable to control the critical dimensions (CDs) of such components. If the critical dimensions of such components are not controlled, some of the components may be shorted together and consequently the semiconductor device being fabricated may not function properly.
In order to control the critical dimensions of these components, the processes used to manufacture these components should have good etch selectivity (to salicides, photoresists, metals, etc.) and high aspect ratios. Prior art processes have the ability to control the critical dimensions within acceptable tolerance ranges for components with dimensions in the range of approximately 0.5-1.0 micron (.mu.). However, as the dimensions decrease below this level, and especially at or below 0.25.mu., these processes do not provide sufficient control over the critical dimensions due to poor etch selectivity and low aspect ratios. Etch selectivity is the ratio of the etch rates of different materials. For example, the etch selectivity for forming a contact may be the ratio of the etch rate of an oxide layer to that of a salicide layer (i.e. oxide: salicide). Aspect ratio is the depth that an etch process can achieve while maintaining the requisite critical dimensions. For example, when forming a contact an etch process with too low an aspect ratio may form the contact too shallow such that the contact may not reach the underlying salicide or polysilicon layer.
FIGS. 1a-1c illustrate the generic (or well known) process steps for photolithographic and masking techniques used to form contacts. It should be noted that the Figures are merely illustrative and have been simplified for clarity purposes and that similar processes may be used to form more complex structures as will be illustrated in later figures. FIG. 1a illustrates a substrate 100 with an oxide layer 110 deposited thereon. In FIG. 1b, a photoresist layer 120 has been spun above oxide layer 110 and is exposed to light through mask layer 130. After photoresist layer 120 has been exposed to light, mask layer 130 is removed and photoresist layer 120 is developed in a developing solution to remove the portions of photoresist layer 120 that were not exposed to light. FIG. 1c illustrates photoresist layer 120 after the soluble portions of photoresist layer 120 have been removed. It should be noted that although the above description describes positive resist techniques, it will be obvious to one with ordinary skill in the art, that negative resist techniques may also be used to form contacts.
Once the soluble portions of photoresist layer 120 have been removed, oxide layer 110 is then etched to form an opening 140, in order to create a contact. The particular etch chemistry and process parameters used not only determine the amount of control over the critical dimensions such as width and length, but also control the aspect ratio and etch selectivity (to salicides, photoresists, metals, etc.) of the contact.
As illustrated in FIG. 1d, prior art processes are unable to control the critical dimensions of opening 140. The sidewalls of opening 140, as shown in FIG. 1d, are not vertical and slope outward at the top of the opening. For packing densities it is ideal to form an opening with vertical sidewalls, i.e. sidewalls with a slope of 90.degree., while keeping the final CD the same as the printed CD. However, typical prior art etch processes only have the ability to form sidewalls with a slope of approximately 85-86.degree., while keeping the final CD the same as the printed CD. Some prior art processes that produce near 90.degree. slope, have final CDs larger than the printed CDs because they give up photoresist selectively to gain the near 90.degree. slope. As illustrated in FIG. 1b, the masks 130 are the same width as the space between the masks, such that the openings 140 (illustrated in FIG. 1d) should be the same width as the spaces in between the openings. For example, openings printed with a width of 0.32.mu. and spaced 0.32.mu. apart should form openings with a width of 0.32.mu. and spaced 0.32.mu. apart. However, due to poor photoresist selectivity, the width of the openings are larger than the masks 130 and consequently the spaces between the openings are smaller. In the example given above for openings printed at 0.32.mu. and spaced at 0.32.mu., using prior art processes the openings may be formed with a width of approximately 0.40.mu. and spaced approximately 0.24.mu. apart.
Vertical sidewalls and adequate space between openings help to maintain the electrical characteristics of the device. Control over the vertical sidewalls as well as the width of the components allow the formation of such components closer to one another without shorting the components together. Thus, as the density of such components on a device increase such changes in the width and the slope of the sidewalls of the opening cannot be tolerated.
Other problems with prior art processes for forming contacts that may affect the electrical performance of a device are illustrated in FIG. 2a. For example, as shown in FIG. 2a, prior art processes not only have problems with the slope of the sidewalls and large opening widths, but also have low aspect ratios. The aspect ratio is the depth that an etch process can achieve while maintaining the requisite critical dimensions. In this case, the aspect ratio is defined as the contact depth divided by the contact opening. As illustrated in FIG. 2a, opening 260 has a high enough aspect ratio to form a contact with salicide layer 230 above gate electrode 220. However, opening 270 has too low an aspect ratio to form a contact with salicide layer 230 above source/drain region 211. Therefore, the electrical connection to source/drain region 211 is never made and the device is defective. This aspect ratio problem is also known as "etch stopping."
Etch stopping, or low aspect ratio, is a phenomena wherein the etch chemistry used to etch the opening form polymer deposits. The build-up of these polymer deposits may degrade the etch process. The amount of polymer formed is a function of the etch time. Thus, the greater the etch time the greater the amount of polymer that is formed. Eventually, the amount of polymer formed may stop or slow the etch process such that the opening being etched may not be formed deep enough. Also, as more and more wafers are etched in a process chamber, for example, in a parallel plate reactor, the greater the build-up of polymer deposits. After a certain amount of time (or after processing a certain number of wafers, for example 400-1000 wafers) the process chamber must be cleaned to remove the polymer deposits such that the etch step may continue in a reliable and efficient manner. Stopping to clean the process chamber takes time and consequently decreases the throughput of the system.
Another problem with prior art processes is also illustrated in FIG. 2a. In addition to the problems with the slope of the sidewalls, large opening widths, and low aspect ratios, prior art processes may also have problems with salicide selectivity. During the etch process used to form opening 260 as shown in FIG. 2a, the etch did not stop on salicide layer 230 and actually punched through the salicide layer to the underlying gate electrode 220. Punchthrough of the salicide layer may degrade the electrical performance of the device. Salicide punchthrough is especially prevalent when prior art processes sacrifice photoresist selectivity to achieve vertical slopes and high aspect ratios.
Some prior art processes utilize "etch stops" to compensate for the poor selectivity of the etch (e.g. etch selectivity for salicides). Etch stops are an additional layer that may be added between for example, a salicide layer and an oxide layer that is made of a material that has a slower etch rate than that of the oxide or salicide. It should be noted and it will be obvious to one with ordinary skill in the art that etch stops may be used between any number of materials and the example of oxide and salicide layers given above is merely illustrative. The use of etch stops require the addition of an extra processing layer above the salicide layer and consequently require additional processing steps. It should be noted that anytime additional processing steps are added, the probability for processing errors during fabrication increases and so to does the cost of the fabrication process.
Thus, what is needed is a method for forming contacts with vertical sidewalls, improved etch selectivity, and submicron dimensions, such that smaller and more dense semiconductor devices may be fabricated and still exhibit good electrical performance. It is also preferable that this method does not require an etch stop layer as to reduce the cost of semiconductor device manufacturing.